The system of the present invention relates to radio communication systems and more particularly to a radio receiver and antenna system which is useful as an electronic counter-counter (ECCM) system.
At the present time, radio communication in a military situation is part of electronic warfare (EW) in which the enemy, or potential enemy, attempts to intercept messages or to prevent the transmission of messages. One widely used method of preventing transmission by radio is to "jam", i.e., disturb the radio communication between a radio transmitter and a radio receiver. Such radio jamming may either be broad band (a wide portion of the radio spectrum) or narrow band (a narrow portion of the radio spectrum).
One broad band method that may be used to jam radio transmission is to produce noise over the entire frequency spectrum that might be used by the radio transmitter. For example, one may modulate the RF signal with a noise source over the range from 30 to 80 MHz, the range most likely used for radio transmission. Such broad band jamming may require a large transmitter having a large power source and may not be effective to curtail shortrange communication, since generally such large jamming equipment would be far removed from the transmitters and receivers which are to be jammed.
In narrow band jamming one attempts to find the exact, or approximate, frequency at which the radio transmission occurs and to jam the transmission at that frequency. For such narrow band jamming one must first find the frequency upon which the transmitter is broadcasting and then tune the jamming radio transmitter to the same frequency and broadcast the noise. Such tuning may be performed manually by turning the dial of the radio receiver until it receives a broadcast, and then tuning the jamming radio transmitter to the same frequency. However, such manual tuning is slow and the transmitting party may be able to complete the message before the jamming broadcast is initiated. In addition, manual tuning depends upon the skill and diligence of personnel.
An alternative to manual narrow band tuning is a system which automatically detects the frequencies being utilized for transmission and automatically tunes a jamming radio transmitter at such frequency. Such automatic devices may operate rapidly and without the use of skilled personnel. However, such automatic devices may be relatively complex, large in size and consequently their placement may be far removed from the battlefield or other location where the transmission occurs. Such narrow band jamming is sometimes called "spot jamming" and may modulate an RF signal with a noise source at the selected frequency.
The jamming of radio transmission and reception is part of the electronic counter measures (ECM) in which the transmitter performing the radio jamming is part of an electronic counter measure system. The avoidance of such ECM radio jamming is obtained by electronic counter-counter measures (ECCM). One type of ECCM device is a "fast-frequency hopping radio" (FFH) utilized in the ultra high frequency range (UHF) or the very high frequency range (VHF). Such a fast-frequency hopping radio rapidly changes the frequency of its broadcasts, and almost simultaneously the frequency of reception by its receivers, in order to avoid a jamming noise signal which may be introduced on its original frequency. By the time the original frequency has been jammed, the fast-frequency hopping radio (FFH) has moved its transmission frequency to a new frequency.
A fast-frequency hopping radio transmission system (FFH) requires that the transmitter and receiver be in synchronism as to the changes in frequency. If the transmitter changes its frequency, to avoid jamming, and the receiver does not change its frequency at the same time to the new frequency, then the message will be lost.
One method of control over the frequency of the receiver by the transmitter, i.e., the selection of the new frequency by the transmitter acting as the master unit and the receiver acting as the slave unit, utilizes a coded message giving the new frequency information (the frequency to which the transmission will be hopped).
Another method of controlling both the transmitter and receiver hop frequencies is the use of identical pseudo random hopping pattern command circuits within both receiver and transmitter. The hopping pattern command circuits must be synchronized in time prior to transmission of a message. This is done by either a time-frequency search of a short, repeated hopping pattern which serves as a preamble, or some form of preset, time-of-day generation of a long hop pattern.
The problem of communications by radio in a battlefield situation may be complicated by other noise sources, in addition to jamming by enemy ECM transmitters. Such noise sources include radio transmission from friendly allied transmitters which arise from lack of coordination, as to frequencies, between various allied forces who may be operating in the same area and on the same frequency.
In addition to the ECCM measures that may be taken with the transmitter and receiver using waveform processing, such as fast frequency hopping radios (FFH) and frequency selective filters, it has also been suggested that the pattern of the receiver's antenna may be controlled to reduce jamming and other noise sources (antenna pattern adaptation). For example, if the location of the transmitter is known and fixed, then a directional high gain antenna may be directed towards the transmitter whose communication it is desired to be received. Even if the transmitter or receiver are moved, it is possible to utilize a highly directional antenna steered, either by hand or automatically, to favor radio reception from the desired transmitter and to reduce reception from jamming transmitters and other noise sources.
An alternative to the use of highly directional antennas is a null-forming antenna system which forms pattern nulls, i.e., non-receiving areas, in the direction of the interference. It has been shown that such antennas may produce a very large rejection of unwanted signals.
The directional ability of the antenna may be either determined by its physical structure or electrically. The physical structure includes its shape, the direction to which it is pointed, and its spacing. In addition, it is known that a directional effect may be obtained electrically using an array of antennas with the radiation pattern of the antenna array being varied, for example, by switching. In addition, the detected RF energy may be processed, i.e., wave form processing, without changing the antenna, so that the antenna array system detects signals from the transmitter whose emissions are desired to be detected and rejects interference by creating null patterns. One type of such RF wave form processing antenna system is called a steerable null antenna processor (SNAP). A typical SNAP system is shown in U.S. Pat. No. 4,298,873. The SNAP system determines the direction of interference and produces antenna nulls in those directions by processing the received RF signals. Such spacial discrimination in the detection of radio transmissions provides a reduction of the noise i.e., the unwanted RF energy to the input port of the receiver. In the SNAP system the control may be either manual or automatic and operates in the 30-80 MHz bandwidth. The SNAP system operates in an antenna pattern forming system using a number of antenna elements forming an antenna array and shifts the phase and adjusts the amplitudes of the RF output of each antenna element. For example, if the antenna array consists of two antenna elements (1 and 2) and the desired signal is S, the noise interference is I, then the SNAP system will attempt, by phase shifting and amplitude adjustment, to cause I.sub.1 vector to cancel the I.sub.2 vector and adds the two signal vectors S.sub.1 and S.sub.2. The pattern of the antenna is not fixed but rather is varied (steered) so that as each pattern is formed it is evaluated and adjusted to achieve maximum performance. For example, the patterns may be automatically changed on a heuristic basis by changing the vector multiplication until the best pattern (highest signal, least noise) is obtained.